<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Title</title>
    <style>
      * {
        margin: 0;
        padding: 0;
        box-sizing: border-box;
      }
      .out_div {
        width: 200px;
        height: 10px;
        border: 1px solid red;
      }
      .in_div {
        width: 50%;
        height: 8px;
        background-color: red;
        transition: all 0.5s ease;
      }
      .out_div:hover .in_div {
        width: 100%;
      }
    </style>
  </head>
  <body>
    <div class="out_div">
      <div class="in_div"></div>
    </div>
  </body>
</html>
